Vacuum seal for electron tubes



March 6, 1962 l. E. MARTIN 3,024,300

VACUUM SEAL FOR ELECTRON TUBES z/M/"ifiiii IN V EN TOR. [kw/w 5 Mir/N 5 Sheets-Shea 2 INVENTOR. E MAW/m arzgzw/i/ l. E. MARTIN March 6', 1962 Original Filed May 9. 1955 March 6, 1962 l. E. MARTIN 3,024,300

. VACUUM SEAL FOR ELECTRON TUBES Original Filed May 9. 1955 s Sh t Sh t a d /f I I I 1 ///,m 1 I .M J /72 lg W m m 5. V

March 6, 1962 I. E. MARTIN 7 3,024,300,

VACUUM SEAL FOR ELECTRON TUBES Original Filed May 9. 1955 5 Sheets-Sheet 4 l n I I I i a IN VEN TOR.

[iv/N6 5 MIN/N March 6, 1962 1. E. MARTIN VACUUM SEAL FOR ELECTRON TUBES Original Filed May 9. 1955 5 Sheets-Sheet 5 5 Mu WM INVENTOR. few/v4 E Main/v United States Patent Office 3,024,300 Patented Mar. 6, 1962 Claims. (Cl. 17450.61)

This invention relates to the art of making vacuum tight seals between electrical insulators and conductors and particularly to the art of making vacuum tight mechanical seals between insulating and conductive portions of the vacuum envelopes of electron tubes. For purposes of simplicity, a vacuum tight seal or sealed condition will sometimes hereinafter be referred to in the specification simply as a seal or sealed. A vacuum tight seal is a joint or seal between two separate envelope members through which the leakage of air intothe envelope is not measurably greater than the leakage through the Walls of the members themselves, so that the seal is capable of maintaining the required vacuum tight condition of an electron tube envelope for an extended period of time.

The present application is a division of applicants copending application Serial Number 507,501, filed May 9, 1955', now abandoned.

Electron tubes are composed of two basic materials, electrically conductive material (e.g., metals) and hard electrically insulating material (e.g., glasses and ceramics), and comprise an envelope within which the electrodes or electron active elements of the tube are arranged in spaced, electrically separated relationship. The conductive material forms the electrodes of the tube and the electrical connections thereto, and the insulating material provides the electrical separation of such electrodes and connections.

Some electron tubes are so constructed that the insulating material forms substantially all of the envelope. Other electron tubes are so constructed that structural portions of the electrodes, that is portions not directly involved in the emission, control, or reception of electrons, form portions of the envelope. In either case, the electrical connections to the electron active portions of the electrodes of the tube must pass through the envelope to allow the connection of external circuitry thereto. Thus, it is necessary that at least a portion of the insulating terial and at least a portion of the conductive material of which an electron tube is constructed be caused to enter into sealed or vacuum tight relationship with each other to form the vacuum tight envelope. It is also necessary that the surface of the conductive portion in contact with the insulating portion exhibit good electrical conductivity and that the seal be capable of remaining vacuum tight at the operating tempertaures of the electron tube.

A primary object of this invention is to provide an improved seal between a hard electrically insulating member and a metal member.

Another object of this invention is to provide an improved mechanical seal between the insulating portions and the conductive portions of the vacuum envelope of an electron tube.

A still further object of this invention is to provide an improved mechanical seal between a ceramic member and a metal member which will remain vacuum tight throughout a temperature cycle.

Still another object of this invention is to provide a seal between a ceramic member and a metal member which will have high electrical conductivity.

Yet another object of this invention is to provide an improved electron tube envelope structure.

Briefly, a seal according to this invention comprises an electrically conductive metal member having a seal surface, a hard electrically insulating member having stress concentrating means providing a seal surface thereon, and means for urging the conductive member and the insulating member toward each other in a direction sub stantially normal to the seal surfaces whereby the seal surface of the insulating member is maintained in intimate contact with the seal surface of the conductive member and the contact region of the seal surfaces is maintained in compression. The stress concentrating means may take the form of any intersection of surfaces or other ridge or shoulder on the insulating member which will make substantially line contact with the conductive member. The method of accomplishing a seal according to this invention is purely mechanical, being efiected by physical force alone, and without any chemical bond between the seal surfaces of the conductive member and the insulating member.

This invention is more completely described hereinafter with reference to the appended five sheets of drawing wherein:

FIG. 1 is a view in cross-section of a ceramic member and a metal member immediately prior to their being brought into sealed relationship in accordance with this invention;

FIG. 2 is a view in cross-section of the ceramic member and the metal member shown in FIG. 1 after they have been brought into sealed relationship in accordance with this invention;

FIG. 3 is a View in cross-section of the seal between the ceramic member and the metal member taken along line 3-3 of FIG. 2 and shows the relationship of the forces and stresses set up there-in;

FIG. 4 is an idealized stress-strain diagram for a ceramic material and for a metal material having high tensile strength;

FIGS. 5, 6, and 7 are cross-sectional views of other embodiments of a seal according to this invention;

FIG. 8 is an enlarged view in cross-section of a seal between a ceramic member and a metal member in vacuum tight relationship in accordance with this invention;

FIG. 9 is an enlarged view in cross-section of a seal between a ceramic member and a metal member in accordance with another embodiment of this invention;

FIGS. 10 and 11 are graphs showing the relationship between a particular hollow cylindrical metal member and a particular hollow cylindrical ceramic member sealed to each other in accordance with this invention in terms of stress, strain, temperature, and the ratio of wall thickness of the members;

FIGS. l2, l3, and 16 are views in cross-section of still other embodiments of this invention;

FIG. 14 is a perspective view, partially in cross-section of a portion of the electron tube shown in FIG. 15, and;

FIG. 15 is a view in elevation of an electron tube constructed in accordance with this invention.

Referring to FIG. 1, a ceramic insulating member 10 and a metallic conductive member 52 are shown which are adapted to form a seal in accordance with this invention. Both the insulating member 10 and the conductive member 12 may be hollow cylinders, the outer diameter of the insulating member 10 being larger than the inner diameter of the conductive member 12. The outer surface 14 of an end portion 1.6 of the insulating member 10 is provided with a tapered surface 18 whereby the outer diameter of the insulating member at the end of the portion 16 is smaller than the inner diameter of the conductive member 12. The intersection of the tapered surface 18 with the cylindrical outer surface 14 forms a sharp ridge or shoulder 19.

The effect of various ratios of the outer diameter of the insulating member 19 to the inner diameter of the conductive member 12 and the effect of various angles of taper of the surface 18 on the end portion 16 of the insulating member will be discussed in greater detail hereinafter. The relative wall thickness of the two members 10 and 12 with respect to each other may vary, as will be discussed hereinafter, but in the embodiment shown, the wall of the insulating member 1%.) is somewhat thicker than that of the COI'ldUCtlVE member 12.

According to one method of forming the seal, the two members 10 and 12 are placed in co-axial, end-to-end relationship, the tapered end portion 16 of the insulating member 10 being thus in position to enter into the conductive member 12. Sufiicient axial forces are then applied to the two members 10 and 12 to force them into vacuum tight relationship as shown in FIG. 2. The end of the conductive member 12 first comes into contact with the tapered outer surface 13 on the end portion 16 of the insulating member 10 and is expanded or flared outwardly as the members 16 and 12 are forced toward each other. The relative axial movement of the members is continued until the shoulder 19 is positioned within the member 12, as shown in FIG. 2. The insulating member 14) is put under compression and the conductive member .12 is put under tension as the members are forced together axially.

Referring to FIG. 3, the application of axial forces to the two members 10 and 12, when properly oriented with respect to each other, results in tensile strain 22 (i.e. actual deformation in the direction indicated) in the conductive member 12, compressive strain 24 (i.e. actual deformation in direction indicated) in the insulating member 10, and compressive stress 26 (i.e. force exerted in the direction indicated) acting mutually between the two members Iii and 12 at the region of contact between the inner surface 28 of the conductive member 12 and the shoulder 19 of the insulating member 10, i.e., at the interface of the seal. The shoulder 19 on the insulating member 10 serves as a means for concentrating the compressive stress 26 in a relatively narrow seal area.

The compressive stress 26 above described causes the two members it) and 12 to enter into such intimate contact at the shoulder 19 that a vacuum-tight seal results. It is believed that substantially molecular contact at the seal area occurs between the two members 10 and 12 thus preventing the passage of gas molecules therebetween. Experiments indicate that no alloying of the materials of which the members iii and 12 are composed occurs, thus a seal according to this invention is a purely mechanical seal as distinguished from a chemical seal such as is produced by the alloying of two materials.

Even after the removal of the axial forces, the compressive stress 26 will continue to be exerted due to the elasticity or resilience of the members it 12, that is, their tendency to return to their unstrained state. Thus, the members it), 12 act as a strain storage means similar to springs in that an applied force causes deformation thereof, which, due to the elasticity of the members it 12 will exert a force after the removal of the applied force which urges the members toward the other. The compressive force 26 exerted by the members 1%, 12 on each other due to such strain storage will maintain the intimate contact at the shoulder 19, thus producing a stable seal upon removal of the applied axial forces.

The mechanical characteristics of the materials used in the formation of a seal as described above will be better understood by referring to FIG. 4- which is an idealized stress-strain diagram for the general types of materials used. In the diagram, the unit stress (i.e. force in pounds per square inch) applied to the materials is plotted along the ordinate of the diagram and the unit strain (i.e. the deformation in inches per inch) is plotted along the abscissa, as is customary in stress-strain diagrams. The dotted line represents the characteristic stress-strain curve of a hard ceramic insulating material such as may be used in accordance with this invention and the solid line represents the characteristic stress-strain curve of a metallic conductive material which may be used in accordance with this invention. Although the two curves are plotted on the same co-ordinates, it should be pointed out that the slopes of the curves are not to be compared directly. In other words, each curve is plotted, assuming different magnitudes of units on the co-ordiuates for the two materials. This fact is pointed out because it is possible that the slope of the curve for the conductive material could be steeper than that for the insulating material, or viceversa, depending on the choice of materials to enter into vacuum tight relationship according to this invention.

Referring to the solid line representing the characteristic stress-strain curve of the conductive material, a number of significant points are marked thereon and labeled A, B, C, D, and B, respectively. Point A on the curve represents the proportional limit of the conductive material, that is, it represents the point below which the strain of the material will be directly proportional to the stress applied to the material and above which such proportionality no longer obtains. Point B is the elastic limit of the material and represents the point below which the stress applied to the material will not result in permanent deformation thereof and above which permanent deformation will occur. Point C is the yield point and represents the point at which continued deformation of the material will result without further application of stress to the material. Point D is the ultimate strength of the material and represents the maximum stress that the material can sustain, calculatedon the basis of the ultimate load the original dimensions. Point E is the breaking point of the material and represents the point at which the material will actually fracture, which point will occur after continued deformation but without further stress than that applied at the point of ultimate strength. These points occur at spaced intervals along the characteristic curve of the material.

Referring to the dotted line representing the characteristic stress-strain curve of the insulating material, it will be seen that a single point represents all of the points mentioned above, that is, the insulating member reaches its proportional limit, its elastic limit, its yield point, its ultimate strength and fractures substantially simultaneously. Thus, so long as the stress applied to the insulating material is not sufficient to fracture the material, it ordinarily will not be permanently deformed but will be strained an amount in proportion to the stress applied.

From the above discussion and a study of FIG. 4, it will be seen that the unit strain storage produced in the members 10 and 12 is limited by the mechanical characteristics of the materials of which they are composed. The unit strain produced in the insulating memher It: may not exceed the elastic limit B of the insulating material since the elastic limit of such material and its fracture point E are substantially the same. On the other hand, if the unit strain in the conductive member exceeds the elastic limit B of the conductive material, appreciable permanent deformation of the conductive member will result with no appreciable increase in the strain storage in the conductive member since only elastic deformation can constitute strain storage. Thus, the unit strain storage possible in the members 10, 12 is limited by the elastic limit B of the materials of which they are composed.

The strain storage in the members 10 and 12 produces the compressive stress 26 which exists between the mernbers 10, 12 after the removal of the axial forces of assembly. However, the compressive stress 26 is dependent upon the total strain storage in the members and not upon the unit strain storage. In other words, since unit stress is measured in pounds per square inch and unit strain is measured in inches per inch, the dimensions of the members 10, 12 will impose limits on the compressive stress 26 that can be obtained therebetween, independently of the characteristics of the materials of which the members 10, 12 are composed. Thus, the compressive stress 26 exerted between a given insulating member 10 and a thin walled conductive member 12, having a high elastic limit B may be the same as the compressive stress 26 exerted between the same insulating member 10 and a thick walled conductive member 12 having a considerably lower elastic limit B.

Referring to FIGS. 2, 5, and 7, the above discussion of the effect of the thickness of members to be sealed according to this invention discloses the extremes possible in forming such seal. One such extreme is indicated by FIG. and includes an initially unflared thin walled metallic cylinder 34 into which a thick walled tapered ceramic insulating member 36 is axially forced resulting in the expanding or flaring of the end 38 of the metallic cylinder 34 as shown. Assuming the extreme case, the insulating member will not be deformed appreciably, substantially the entire deformation or strain storage produced being in the metallic member. The other extreme is indicated by FIG. 7 and involves the use of a strong thick walled metallic cylinder ill having an initially flared end 42 (or beveled inner surface at such end 42) into which a tapered insulating member 44 is axially forced. Again assuming the extreme case, the conductive member 40 will not be deformed appreciably, substantially all of the deformation or strain storage in the system being in the insulating member 44.

According to the embodiment shown in FIG. 2, the intermediate case is represented. That is. both the conductive member and the insulating member will be deformed or strained an amount dependent upon the materials and the ratio of thickness of such members.

According to the preferred practice of this invention, the materials of which the members are composed and the thickness ratio between the members are selected such that the conductive member will be strained slightly beyond its elastic limit so that the tolerances in the size of the members will not be critical, since the compressive stress 26 produced will always be limited by the elastic limit of the conductive member 12, as described above.

The effect of the relative thickness of the members upon the amount of strain storage possible in a seal according to this invention will be better understood by referring to FIG. 10. FIG. is a graph in which a family of curves is plotted for a specific seal between an insulating member of a particular material and a conductive member of a particular material in the form of cylinders having a contact region of a particular radius. It can be shown by the algebraic manipulation of expressions found in chapter 12 of a book entitled, Formulas for Stress and Strain by Raymond J. Roark, Mc- Graw Hill Book Company, New York, 1943, that the force in pounds per inch on the periphery of a loaded cylinder is approximately as follows:

where P is a constant dependent upon the locus or position of loading of the cylinder, V is Poissons ratio for the material of which the cylinder is composed, E is the modulus of elasticity for such material, I is the wall thickness of the cylinder, R is the mean radius of the cylinder, and AR is the actual change in mean radius of the cylinder.

Since the force acting on the two cylinders in this case is the same, that is, equal to the compressive stress 26 acting between the two members, the following relationship can be established:

where the subscript c identifies the terms applicable to the insulating member and the subscript m identifies the terms applicable to the conductive member. However, since the materials of which the members are composed is to be held constant as well as the radius of the contact region and the position of loading, the only variables remaining in the above expression are the changes in radius (AR) of the two members and the thicknesses (t) of the two members. These variables are plotted in the graph shown in FIG. 10. In the graph, the unit change in radius of the insulating member is plotted along the ordinate and the relative thickness of the members is plotted along the abscissa (the thickness of the insulating member being assumed to be unity). .Each of the solid lines in the graph represents a constant unit change in radius of the conductive member as... m

It will also be seen that a given total unit change in radius may be obtained at any ratio of the thickness of the members but with different proportions thereof in each of the members,

As explained above, the shoulders or ridges formed by the tapers on the end portions of the insulating members it 36, and 44 serve primarily as stress concentration means. The contact between the members will occur over a narrow area and substantially all of the stresses produced in forming the seal will be concentrated at such narrow area. The effect of such stress concentration is to enable the intimate vacuum-tight contact between the members In and i2, 34 and 36, 42 and 44 to be obtained through the use of practical values of axial forces. Since the amount of force necessary to produce the desired contact increases with the area of contact between the members, the ideal would be a line contact which is approximated in the embodiments shown in FIGS. 2, 5, and 7.

Although it is preferable to use a straight taper which forms a relatively sharp ridge or shoulder 19, such as is shown in FIGS. 2, 5, and 7, the stress concentrating means may take the form of an arcuate taper 32 as shown in FIG. 6, in accordance with this invention. As shown in FIG. 6, an end 24 of the metallic conductive member 12 preferably may be provided with an initial arcuate flare of given radius. An end portion 16 of the ceramic insulating member 10 is provided with an arcuate taper 32 having av radius less than the radius of the flare of the end 2d of the conductive member 12. Thus, the arcuate taper 32 of the insulating member 10 may be brought into tangential contact with the flared end portion 2t) of the conductive member 12 and the resulting compressive stress will result in a seal according to this invention.

An additional advantage of the taper on the end of the insulating member, whether it be straight or arcuate, is that it spaces the incidence of stress upon the insulating member from the end thereof. By spacing such incidence of stress from the end of the insulating member, the bending moments induced in the member by such stress are minimized, reducing the possibility of fracture of the insulating member by the tensional forces resulting from such bending moments. In effect, the bending moments induced by a compressive force applied at the end of the insulating member will produce tension forces varying in magnitude as a damped sine function axially along the insulating member. However, if a portion of the insulating member extends beyond the point at which the compressive force is applied, such extending portion will introduce a new bending moment which will partially cancel the bending moment induced by the compressive force.

The angle of the taper on the ends of the insulating member may vary according to the materials used and the application for which the seal is designed. In successfully tested seals according to this invention, the taper on the insulating member formed an angle of about 7 with respect to the body of the insulating member. However, such angle is not critical and may be determined from the relative diameter of the two members, the materials of which they are composed, and bending moment considerations, the most important function of the taper being to provide the small area contact between the members.

According to this invention the materials of which the metallic conductive members are composed may not always be the best possible electrical conductors. However, seals made according to this invention may be adapted to exhibit higher radio frequency conductivity than any other type of seal by simply providing a layer or coating 46 of high conductivity material such as copper, gold or silver on the metallic member as shown in FIGS. 5, 6, 7, 8, and 9. In each case, the layer 46 should be considered as effectively a part of the metallic member.

Due to the fact that a seal according to this invention is purely mechanical, the electrical properties of such high conductivity layer or coating 46 will not be changed by the sealing process since no alloying of such coating 46 will occur. In all other types of seals, which may be classified broadly as chemical seals, some alloying of any high conductivity coating that may be used will occur which will greatly reduce the electrical conductivity of the coating at the seal.

The importance of the high conductivity coating 46 is considerable where the current to be passed through the seal is a radio frequency current. This is due to the fact that radio frequency cunents tend to flow in the surface layer of conductors. Since such surface layer of a conductive member according to this invention is provided by a high conductivity coating 46 whose conductivity is not lessened by alloying in making the seal, the electrical resistance of such conductive member to the flow of radio frequency currents will be equal to that of the coating 46, which may be silver or copper as stated above.

The use of the conductive coating 46 in seals according to this invention has a further advantage in that high conductivity materials such as silver, copper or gold are readily deformable or malleable. Thus, the coating 46 will be easily deformed under the influence of the compressive stress 26 exerted between the members to be sealed. Such deformability makes it possible to obtain intimate contact between the members with lower applied forces than would otherwise be necessary.

FIGS. 8 and 9 are enlarged views in cross-section of the seal area of seals accomplished according to thisinvention. FIG. 9 illustrates that the deformable coating 46 on the metallic member l2 has been scraped by the ceramic insulating member it and caused to flow to form a thickened portion or gasket 48 between the insulating member 10 and the metallic member 12. Such scraping or wiping action aids in achieving the necessary intimate contact at the shoulder 50 by causing flow of the deformable material 46, spanning any porosities which may exist in the sealing surface of the insulating member at the shoulder Sil and thus insuring continuous contact between the two members about the periphery thereof. The presence of the gasket 48 is usually an indication of a good seal in that it indicates even contact and considerable stress between the two members 10, 12. Although successful seals have been made between ceramic insulating members 1-0 and steel conductive members 12 without a deformable coating 46 on such steel conductive member 12 by highly polishing the seal surfaces, the presence of the deformable coating enables the formation of the seal at much lower applied forces and without highly polishing the seal surfaces.

FIG. 8 shows a seal which may be accomplished according to a number of different methods according to this invention. For example, the seal shown in FIG. 8 may be an enlargement of the seal shown in FIG. 7. That is, the conductive member 40 is heavy or thick and is provided with an initial flare 42. Thus, the bulk of the strain produced in accomplishing the seal takes place in the insulating member 44 and the wiping action previously described with respect to FIG. 9 is reduced to a minimum. However, some deformation of the inner surface of the conductive member occurs which is enhanced by the presence of the deformable coating 46 forming a part thereof. The tapers on the insulating member 44 and on the conductive member 40 co-act to concentrate the stress at substantially a line contact.

It will be noted that the members ill, 12 or n), 44 themselves comprise means for urging such members toward each other in a direction substantially normal to the seal surfaces to maintain the seal surfaces under compression and accomplish the seal.

In making seals according to this invention, ceramics containing a high percentage of aluminum oxide have been successfully used as the insulating member because of their high strength in compression and excellent electrical properties. For example, ceramics having a content of more than 50.26% aluminum oxide, up to 30% of an oxide of manganese, from 2.5% to 2.8% silica, and up to 4% of at least one alkaline earth of the group consisting of the oxides of beryllium, magnesium, zinc, calcium, strontium, cadmium and barium may be used. Other insulating materials could be used if the members are properly designed to make allowance for the mechanical characteristics of such materials. The conductive member in successfully tested seals according to this invention has been composed alternatively of a variety of metals including Cold Rolled Steel (SAE l0l0), lnconel, Kovar, 883 Tool Steel, 42 Metal, and copper. in some successfully tested seals, the conductive member has been provided with a coating of copper or silver, and in other tests a gasket cylinder made of copper foil was interposed between the insulating member and the conductive member.

For example, thc following data is taken from a specific successful test. The insulating member comprised a cylinder of the ceramic above described having an outer diameter of 2.481 inches and a wall thickness of approximately 0.250 inch. The conductive member comprised a composite cylinder having an inner diameter of 2.428 inches and consisting of a cold rolled steel cylinder having a. wall thickness of 0.070 inch and a copper gasket cylinder having a wall thickness of 0.026 inch placed as a lining inside the cold rolled steel cylinder. The ceramic cylinder was provided with a 7 taper ground on one end thereof and intercepting the previously ground outer diameter of the cylinder at about .250 inch from such end. The ceramic cylinder was pressed into the corn =sitc metallic cylinder to a distance of 0.63 inch, requiring approximately 12,000 pounds of axial force and resulting in a total deformation of the members of 0.053 inch.

The exact nature of the seal obtained according to this invention is not fully understood and the claims are not intended to be limited by theories of operation herein set forth. The results of tests indicate that the compressive stress necessary to produce a good seal depends upon the relative hardness of the two materials and the surface conditions of the members at the seal area. Calculations from empirical work on a particular seal indicate that vacuum tight seals have been formed with forces of 800 pounds per lineal inch of contact at room temperature, but this figure is not necessarily applicable to other seals.

Tests have established that once intimate vacuum-tight contact between the conductive member and the insulating member has been achieved, very little compressive stress is necessary to maintain such contact at substantially the same temperature. in other words, although considerable compressive stress is necessary to cause the seal surface of the conductive member to enter into intimate contact with the seal surface of the insulating member, once such contact is obtained, the compressive stress necessary to maintain the contact is greatly reduced.

However, as is well known in the prior art, a seal between a conductive member and an insulating member is difiicult to maintain throughout a large temperature cycle due to the fact that conductive members normally exhibit greater expansion under the influence of heat than do insulating materials. In other words, it a seal is accornplished at a given temperature, and then subjected to considerable heat, the conductive member will expand more than the insulating member and such expansion differential, if sufficient, may destroy the seal between the two members.

In order to maintain a seal according to this invention throughout a given temperature cycle, the members to be sealed may be designed so that the strain storage therein will be only partially relieved by the unequal expansion of the members upon heating. In other words, the members may be so designed that when the seal therebetween is accomplished, the strain produced in the members will be sullicient to produce a. compressive stress 26 which exceeds the minimum compressive stress necessary to maintain the intimate contact between the members. This, of course. is not practicable in prior art fusion seals, which should be without excessive stress in any indirection at all times.

The total amount of strain necessary to maintain compressive stress between the members may be expressed as follows:

represent the unit change in radius of a cylindrical insulating member and a cylindrical metallic member respectively, AT represents the change in temperature to which the seal will. be subjected, and K and K represent the coefficients of thermal expansion of. the conductive member and the same insulating member, respectively, mentioned with respect to FIG. 10. In the above expres sion, it is assumed that the coeflicient of thermal expansion of the conductive member is larger than that of the insulating member.

Referring to FIG. 11, the above Expression 3 along with Expression 2 enables the plotting of a graph showing the relationship between the thickness of the members and the temperature range to which a seal therebetween may be subjected. The solid curves in FlG. 11 are the loci of constant strain in the conductive member, whereas the broken curves are the loci of constant strain in the insulating member. A study of FIG. 11 will reveal that for a given temperature range AT a great variety of thickness ratios may be used with a corresponding proportionrnent of the total strain between the two members. Such study will also reveal that for a given strain in the conductive member the temperature range which the seal will withstand may be extended by increasing the ratio of the thickness of the conductive member to the thickness of he insulating member.

The above brief discussion of the design of seals according to this invention has been simplified to point out important features thereof. Other design features will be 10 apparent to those skilled in the art and include the fatigue characteristics, the creep characteristics, and the hot strength of the materials used.

The discussion herein has thus far been limited to embodiments of the invention in which the conductive member surrounds the insulating member. In FIG. 12 a seal in accordance with this invention is formed with the ceramic insulating member 51 surrounding the metallic conductive member 52. The hollow insulating cylinder 51 is provided with a tapered surface 53 at the end thereof on its inner surface and a hollow metallic cylinder 52 having an outer diameter slightly greater than the inner diameter of the insulating cylinder 51 is forced into the insulating cylinder 51 by the application of axial forces to the members 51 and 52. The ratio of thickness of the members 51 and 52 must be adjusted so that the insulating member 51 can withstand the tension strain induced therein when suflicient compression stress exists between the members to form a sea].

In designing the embodiment shown in FIG. 12 to withstand a temperature cycle some features of inverse nature to those hereinabove mentioned must be considered. Since the conductive member 52 is arranged internally of the insulating member 51 and the coetlicient of thermal expansion of the conductive member 52 will usually be greater than that of the insulating member 51, the compression stress between the members 51 and 52 will increase with the application of heat. Thus, it is necessary to design the members 51 and 52 such that sutiicicnt compression stress will exist therebetween at the minimum temperature and yet they will be able to withstand the increased compression stress at the higher temperature.

It should be noted at this point that in either the embodiment shown in FIGS. 1 and 2 or the embodiment shown in FIG. 12, the material of which the members 1% and 12 or 51 and 52 respectively are composed could be so selected that the coefficient of thermal expansion of the conductive member 12 or 52 is less than that of the insulating member 10 or 51. It is believed that the discussion hereinabove of the temperature problems is sufiicient to teach one skilled in the art the fundamental consideration which would apply to either case in either embodiment.

H6. 13 shows an embodiment of this invention incorporating the features of both the embodiment described with respect to FIGS. 1 and 2 and the embodiment described with respect to FIG. 12. A ceramic insulating ring or cylinder 54 is wedged between two metallic conductive cylinders 55 and 56. The first conductive cylinder 55 is of larger diameter than the second conductive cylinder 56 and the conductive cylinders 55 and 56 are arranged in concentric relationship in registry at one end. The insulating ring 54 has a wall thickness slightly greater than the difference in diameter of the two conductive cylinders 55 and 56 and is provided with tapered surfaces on both its inner and outer surfaces in accordance with this invention. The insulating ring 54 and the conductive cylinders 55 and 56 are subjected to axial forces to wedge the insulating ring 54 between the conductive cylinders 55 and 56, as shown in FIG, 13. Such wedging, as hereinbefore described, produces a seal between each of the conductive cylinders 55 and 56 and the insulating ring 54.

From the discussion hereinabove, it will be apparent that the insulating ring 54 and the conductive cylinders 55 and 56 may be designed so that the first conductive cylinder 55 will maintain the insulating ring 54 in compression at all times and at all temperatures normally encountered, thus adapting the embodiment shown in FIG. 13 to withstand a temperature cycle. One Way in which the above may be easily accomplished is to make the wall thickness of the outer conductive cylinder 55 thicker than the wall thickness of the inner conductive cylinder 56. Other methods, such as the proper adjustmeat of the relative diameters of the members are equally eifective. Thus, at the minimum temperature to which the seal will be exposed, considerably more compressive stress will exist between the outer conductive cylinder 55 and the insulating ring 54 than between the inner conductive cylinder 56 and the insulating ring 54 thus placing the insulating ring 54 in compression. When the seal is subjected to heat, and assuming that the conductive cylinders 55 and 56 have a higher coeflicient of thermal expansion than the insulating ring 54, the expansion of such conductive cylinders 55 and 56 will act only to relieve the initial compression in the insulating ring 54. Thus, it is possible to so design the seal that the insulating ring 54 is never placed in tension, which is preferable since hard insulating materials are notorious for their poor strength in tension.

FIG. 16 shows the embodiment of FIG. 13 further adapted to provide an additional metallic conductive member 57 extending through a seal in accordance with this invention. Two ceramic insulating rings or cylinders 58, one having an inner diameter slightly larger than the outer diameter of the other, are arranged in concentric relationship in registry at one end. A thin metallic conductive cylinder 57 having a wall thickness approximately equal to the spacing between such insulating rings 58 is sandwiched therebetween. The outer diameter of the larger insulating ring 58 and the inner diameter of the smaller insulating ring 58 are provided with tapers in accordance with this invention. The inner surface of the larger insulating ring 58 and the outer surface of the smaller insulating ring 53 are each provided with a protruding sharp ridge or shoulder 59 to provide stress concentrating means in accordance with this invention similar to that provided by the tapered surfaces, The insulating rings 58 and the thin walled conductive cylinder sandwiched therebetween are wedged between inner and outer conductive cylinders '5 and 56 as described with respect to FIG. 13. Since the insulating rings 58 are maintained in compression by outer conductive cylinder 55, the sharp ridge or shoulders 59 on the insulating rings will be forced into and maintained in intimate contact with the inner and outer surfaces of the thin walled conductive cylinder 57.

The relative wall thickness of the Conductive cylinders 55, 56, and 57 may be adjusted so that the insulating rings 58 will be maintained in compression throughout a given temperature cycle by making the outer cylinder 55 considerably thicker than the inner cylinder 56 and the middle or thin walled cylinder 57 enough thinner than the other two cylinders 55 and 56 so that it will be substantially flaccid and will be dominated by such other cylinders 55 and 56. Thus, a third conductive member may be added to the embodiment of FIG. 13 as shown in FIG. 16 with a minimum of additional design con siderations.

FIGS. 14 and illustrate an electron tube 66 according to this invention. FIG. 15 shows the electron tube 66 to comprise a metallic, water cooled, cylindrical anode assembly 62 and a metallic cylindrical cathode assembly 64 which form portions of the envelope of the tube The remainder of the envelope of the tube 66 is formed by the cathode terminal assembly 66, a hollow ceramic insulating cylinder 68 extending between the anode assembly 62 and the cathode terminal assembly 66. a grid terminal assembly 70, a first ceramic insulating ring 72 (not shown in FIG. 15) extending between the cathode terminal assembly 66 and the grid terminal assembly 76 and a second ceramic insulating ring 74 (not shown in FIG. 15) extending between the grid terminal assembly 70 and the cathode assembly 64.

As shown in FIG. 14, the anode assembly 62 comprises a hollow cylindrical, metallic anode 76 the inner surface of which serves as the electron active portion thereof and the outer surface of which is provided with passageways 78 to aid in the water cooling thereof. A water cooling jacket 80 surrounds the anode 76 and is sealed to one end of the anode 76 to provide for the circulation of water around the anode 76 and through the passageways 78 therein. An anode terminal flange 82 comprising an annular metallic member of inverted U- shape cross-section surrounds the water cooling jacket 80, one leg of such inverted U-shape being sealed to the outer surface of the jacket 80.

The cathode assembly 64 comprises a hollow cylindrical metallic beam former block 84, one end of which is positioned coaxially within the anode assembly 62 and is provided with a plurality of longitudinally extending slots 86 on its outer surface. A directly heated cathode filament 88 extends within and along each of the slots 86 in the beam former block 84 and is attached at one end directly to such beam former block 84. The other end of the beam former block 84 forms a terminal through which heating voltage may be connected to the ends of the filaments attached thereto. The other ends of the filaments 83 extend beyond said one end of the beam former block 34 and are connected to a pantographic supporting-device (not shown) which is in turn connected to another filament terminal (not shown) which extends co-axially within the beam former block '84 and beyond the lower end thereof to form the other connection for the filament heating voltage.

Between the slots 86 in the beam former block 84 and attached directly to the beam former block 84 are T- shaped shielding members 90, the cross arms of such T-shape members extending transversely between adjacent slots 86. The T-shaped shielding members 90 are pr0- vided with extensions or tabs 92 at the lower end thereof which are electrically connected to the cathode terminal assembly 66. Thus, the cathode terminal assembly 66 serves as one of the radio frequency terminals for the electron tube 60.

Grid rods 94 extend longitudinally along the sides of the slots 86 under the cross-arms of the shielding members 96 and are held in a pantographic grid support 96 at each end (only one pantographic grid support is shown in FIG. 14). The pantographic grid support 96 shown in FIG. 14 constitutes a part of the grid terminal assembly 76 providing the electrical connection between such assembly 70 and the grid rods 94.

The insulating cylinder 68 extends between the anode terminal flange 82 and a sealing flange 98 which constitutes a part of the cathode terminal assembly 66. The outer diameter of the insulating cylinder 68 and the inner diameters of the anode terminal flange 82 and the cathode terminal sealing flange 98 are designed to accomplish a seal according to this invention. The insulating cylinder 68 is provided with a taper on its outer surface at each end thereof. Such tapers are brought into contact with the anode terminal flange 82 and the cathode terminal seal flange 93, respectively, to accomplish the seals.

The first insulating ring 72 is wedged between a first sealing cylinder 1% on the cathode terminal assembly 66 and a second sealing cylinder 162 on the grid terminal assembly 76. The second insulating ring 74 is wedged between a third sealing cylinder 104 which is attached to the grid terminal assembly 76 and a fourth sealing cylinder 166 which is attached to the cathode assembly 64.

The mechanical compression seals between the insulating cylinder 68 and the anode terminal flange 82 and cathode terminal flange 98 are substantially identical to the embodiments of this invention described with respect to FIGS. 1 and 2. Similarly, the first. insulating ring 72 with the first sealing cylinder MK; and the second sealing cylinder 162, and the second insulating ring 74 with the third sealing cylinder 104 and the fourth sealing cylinder 196 form seals that are substantially identical to the embodiment of this invention described with respect to PEG. 13.

It is the presence of such mechanical compression seals according to this invention which distingmish the electron tube 60 from the electron tubes described in US. Patents 2,544,664 and 2,636,142. These patents may be referred to for a more detailed description of the general electrode arrangement of the tube 60 of FIGS. 14 and 15.

In assembling the electron tube 60 shown, the anode assembly 62 is constructed separately from the cathode assembly 64. The grid terminal assembly 70 and the cathode terminal assembly 66 with the associated insulating rings 72, 74 are then placed in proper orientation with respect to each other and the insulating rings 72, 74 and assemblies 64, 66, 70 subjected to axial forces to produce seals in accordance with this invention. Finally, the anode assembly 62 and cathode assembly '64 to which the grid assembly 70 and the cathode terminal assembly 66 have been attached are brought into proper orientation with the insulating cylinder 68 and subjected to axial forces to produce seals according to this invention with such insulating cylinder 68.

Since all metallic parts of the tube 60 carrying high frequency currents may be coated or otherwise provided with a surface layer of high conductivity material, it will be apparent that high conductivity paths for high frequency currents are provided to the various electrodes of the electron tube. That is, since the seals between the insulating members 68, 72, and 74 and the conductive portions of the tube are made in accordance with this invention, a minimum of resistance will be displayed to high frequency current flowing along the surfaces of such conductive portions and to the electrodes of the electron tube. Also, a study of the drawing will reveal the clean construction and ease of fabrication which results from the use of seals according to this invention.

What is claimed is:

1. An electron tube envelope comprising a first tubular metallic member having a portion of given external dimension and another portion decreasing from said given external dimension to a smaller external dimension, a second tubular metallic member having a portion of larger internal dimension than said given external dimension of said first metallic member and another portion increasing from said larger internal dimension to a still larger internal dimension, and a tubular ceramic member of high compressive strength having a mean dimension intermediate said given external dimension of said first metallic member and said larger internal dimension of said second metallic member, a first portion of said ceramic member having a given wall thickness greater than the difference between said given external dimension of said first metallic member and said larger internal dimension of said second metallic member and a second portion of said ceramic member having a wall thickness decreasing from said given wall thickness, said first metallic member and said second metallic member being in concentric relationship with said ceramic member wedged therebetween, said ceramic member having sharp annular ridges thereon in sufiicient intimate radial compressive substantially line contact with said metallic members to maintain vacuum tight seals therebetween solely by virtue of said intimate compressive contact, at least the seal surface of said metallic members being more deformable than said ceramic members, each of said seal surfaces being provided by a narrow annular sealing gasket formed by fiow of the material of said metallic member during axial assembly of said members.

2. An electron tube envelope comprising a first metallic cylinder having a portion of given external diameter and another portion decreasing from said given external diameter to a smaller external diameter, a second metallic cylinder having an internal diameter larger than said .givcn external diameter of said first metallic cylinder, a

third metallic cylinder having a portion of larger internal diameter than the external diameter of said second metallic cylinder and another portion increasing from said larger internal diameters, said second metallic cylinder having a wall thickness substantially less than the wall thickness of said first metallic cylinder, said first metallic cylinder having a wall thickness substantially less than the wall thickness of said third metallic cylinder, a first ceramic cylinder of high compressive strength having a mean diameter intermediate said given external diameter of said first metallic cylinder and said internal diameter of said second metallic cylinder, a second ceramic cylinder of high compressive strength having a mean diameter intermediate said external diameter of said second metallic cylinder and said larger internal diameter of said third metallic cylinder, said first ceramic cylinder having a maximum wall thickness greater than the difference between said given external diameter of said first metallic cylinder and said internal diameter of said second metallic cylinder, said second ceramic cylinder having a maximum wall thickness greater than the difference between said external diameter of said second metallic cylinder and said larger internal diameter of said third metallic cylinder, said ceramic cylinders being provided with stress concentrating means in the form of sharp peripheral ridges about their inner and outer surfaces, said first ceramic cylinder being wedged between said first metallic cylinder and said second metallic cylinder, said second ceramic cylinder being wedged between said second metallic cylinder and said third metallic cylinder, said first ceramic cylinder being in suificient intimate radial compressive contact with said first metallic cylinder and said second metallic cylinder along its said ridges only and said second ceramic cylinder being in suflicient intimate radial compressive contact with said second metallic cylinder and said third metallic cylinder along its said ridges only to effect and maintain vacuum tight seals between said cylinders solely by virtue of said intimate compressive contact, at least the seal surfaces of said metallic cylinders being more deformable than said ceramic cylinders, each of said seal surfaces being provided by a narrow annular sealing gasket formed by flow of the material of said metallic cylinder during axial assembly of said cylinders.

3. An electron tube comprising an evacuated envelope, an electrode contained within said envelope, a first annular ceramic insulating member and a second annular ceramic insulating member of high compressive strength forming parts of said envelope, and an annular metallic lead-in member sealed between said first ceramic member and said second ceramic member and electrically connected to said electrode, said lead-in member comprising a first sealing flange and a second sealing flange, said first sealing flange having a portion of internal diameter smaller than the external diameter of said first ceramic member and another portion of increasing diameter, said second sealing flange having a portion of internal diameter smaller than the external diameter of said second ceramic member and another portion of increasing diameter, the external surfaces of said ceramic members each being provided with stress concentrating means in the form of a sharp peripheral ridge thereon, said other portion of said first sealing flange being in intimate radial compressive contact with the ridge of said first ceramic member producing suflicient tension strain in Said first sealing flange and compression strain in said first insulating member to effect and maintain a vacuum tight seal therebetween, said other portion of said second sealing flange being in intimate radial compressive contact with the ridge of said second ceramic member producing sufiicient tension strain in said second sealing flange and compression strain in said second insulating member to effect and maintain a vacuum tight seal therebetween, at least the seal surfaces of said metallic member being more deformable than said ceramic members, each of said seal surfaces being provided by a narrow sealing gasket formed by flow of the material of said metallic member during axial assembly of said members.

4. An electron tube comprising an evacuated envelope,

an electrode contained within said envelope, a first hollow cylindrical ceramic member and a second hollow cylindrical ceramic member of high compressive strength forming parts of said envelope, and a hollow cylindrical electrically conductive lead-in member sealed between said first ceramic member and said second ceramic memher said lead-in member forming a part of said envelope and being electrically connected to said electrode, said lead-in member comprising a first sealing flange and a second sealing flange, said first sealing flange having a portion of external diameter larger than the internal diameter of said first ceramic cylinder and another portion of decreasing external diameter, said second sealing flange having a portion of smaller internal diameter than the external diameter of said second ceramic member and another portion of increasing internal diameter, the internal surface of said first ceramic cylinder being provided with stress concentrating means in the form of a sharp peripheral ridge thereon, the external surface of said second ceramic cylinder being provided with stress concentrating means in the form of a sharp peripheral ridge thereon, the external surface of said other portion of said first sealing flange being in sufficient intimate compressive contact with the ridge of said first ceramic cylinder to produce and maintain a vacuum tight seal therebetween solely by virtue of said intimate radial compressive contact, the internal surface of said other portion of said second sealing flange being in sufficient intimate radial compressive contact with the ridge of said second ceramic cylinder to produce and maintain a vacuum tight seal therebetween solely by virtue of said intimate radial compressive contact at least the seal surfaces of said metallic member being more deformable than said ceramic members, each of said seal surfaces being provided by a narrow sealing gasket formed by flow of the material of said metallic member during axial assembly of said members.

5. A vacuum tight envelope comprising a hollow cylindrical first member of high tensile strength and a cylindrical second member of high compressive strength telescoped partly within said first member, one of said members being of hard refractory insulating material and the other member being of metal, said insulating member having stress concentrating means thereon in the form of an annular ridge providing a narrow seal surface, the metal member having an annular seal surface telescoped with and in intimate substantially line contact with said annular ridge, at least the seal surface of said metal member being more deformable than said insulating member, said members being maintained solely by said telescopic arrangement under sufficient tensile and compressive strain, respectively, to produce suflicient radial compression at said seal surfaces to maintain a vacuum tight seal therebetween at said seal surfaces over an extended period of time without fracture of said insulating member, said seal surface of said metal member being provided by a narrow annular sealing gasket formed by flow of the material of said metal member during axial assembly of said members.

6. A vacuum tight envelope comprising a hollow cylindrical metal member of high tensile strength and a cylindrical hard refractory insulating member of high compressive strength telescoped partly within said metal member, said insulating member having stress concentrating means thereon in the form of an annular external ridge providing a narrow seal surface, said metal member having an annular inner seal surface surrounding and in intimate substantially line contact with said ridge, at least the seal surface of said metal member being more deformable than said insulating member, said members being maintained solely by said telescopic arrangement under suificient tensile and compressive strain, respectively, to produce sufiicient radial compression at said seal surfaces to maintain a vacuum tight seal therebetween at said seal 16 surfaces over an extended period of time without fracture of said insulating member, said seal surface of said metal member being provided by a narrow annular sealing gasket formed by flow of the material of said metal member during axial assembly of said members.

7. A vacuum tight envelope comprising a hollow cylindrical metal member of high tensile strength and a cylindrical hard refractory insulating member of high compressive strength having an end telescoped partly within one end of said metal member, said end of said insulating member being formed with an annular surface tapered at a given small angle with respect to the axis thereof and intersecting the cylindrical surface of said member to form an annular sharp ridge providing a very narrow seal surface, said end of said metal member having an annular inner seal surface flared outwardly at a given angle less than the first-named angle, said inner seal surface surrounding and in intimate substantially line contact with said ridge, at least the seal surface of said metal member being more deformable than said insulating member, said members being maintained solely by said telescopic arrangement under sufiicient tensile and compressive strain, respectively, to produce sufficient radial compression at said seal surfaces to maintain a vacuum tight seal therebetween at said seal surfaces over an extended period of time without fracture of said insulating member, said seal surface of said metal member being provided by a narrow annular sealing gasket formed by flow of the material of said metal member during axial assembly of said members.

8. An envelope as in claim 7, wherein said metal member includes a thin copper lining, at least at said inner seal surface.

9. An envelope as in claim 7, wherein metal member consists of a thin hollow cylinder of steel with a thin deformable layer forming said inner seal surface.

10. An envelope as in claim 9 wherein said given small angle is about 7.

11. An envelope as in claim 7, wherein said insulating member is made of a ceramic material having high percentage of aluminum oxide.

12. An envelope as in claim 7, wherein said insulating member is a hollow cylindrical ceramic member having a wall thickness that is small compared to its diameter.

13. An envelope as in claim 12, wherein said metal member has a wall thickness substantially smaller than the wall thickness of said ceramic member.

14. An envelope as in claim 7, wherein said metal member is strained slightly beyond the elastic limit of the metal.

15. A vacuum tight envelope as in claim 7 adapted to be subjected to relatively wide temperature range, wherein the coefficient of expansion of said metal member is higher than that of said insulating member, and said members are maintained under sufiicient tensile and compression strain, in excess of the minimum strain required to maintain a vacuum tight seal between said members, to maintain said seal vacuum tight over said range.

References Cited in the file of this patent UNETED STATES PATENTS 881,060 Cook Mar. 3, 1908 1,066,290 Kraus July 1, 1913 1,865,016 Kirkpatrick June 28, 1932 1,896,261 True Feb. 7, 1933 2,121,600 Knowles June 21, 1938 2,250,355 Bruck July 22, 1941 2,271,657 Miller Feb. 3, 1942 2,347,051 Heron Apr. 18, 1944 2,445,777 Hahn July 27, 1948 2.464308 Volkmann Mar. 22, 1949 2,788,994 Van de Wateren Apr. 16, 1957 

